Method for operating a working machine and a working machine with an improved transmission line

ABSTRACT

A working machine and a method for operating the same are provided. The working machine is provided with: a power source and a plurality of driving wheels; a working hydraulic system including at least one hydraulic pump powered by the power source for moving an implement on the working machine and/or for steering the working machine; a transmission line arranged between the power source and the driving wheels for transmitting torque from the power source to the driving wheels; and a transmission unit arranged in the transmission line for reducing the mechanical interaction between the power source and the driving wheels. The method includes: detecting at least one operational parameter indicative of a working condition of the working machine; determining if the characteristic of the transmission unit should be altered on the basis of a magnitude of the detected operational parameter; altering the characteristic of the transmission unit if it is determined to be required.

BACKGROUND AND SUMMARY

The invention relates to a method for operating a working machine and aworking machine.

The invention is applicable on working machines within the field ofindustrial construction machines, in particular wheel loaders. Thus, theinvention will be described with respect to a wheel loader. However, theinvention is by no means limited to a particular working machine. On thecontrary, the invention may be used in a plurality of heavy workingmachines, e.g. articulated haulers, trucks, bulldozers and excavators.

Wheel loaders are generally provided with an internal combustion engine,a transmission line, a gearbox, driving wheels and a working hydraulicsystem.

The combustion engine provides power to the different functions of thewheel loader. In particular, the combustion engine provides power to thetransmission line and to the working hydraulic system of the wheelloader.

The transmission line transfers torque from the combustion engine to thegearbox, which in turn provides torque to the driving wheels of theloader. In particular, the gearbox provides different gear ratios forvarying the speed of the driving wheels and for changing between forwardand backward driving direction of the wheels.

The working hydraulic system is used for lifting operations and/or forsteering the wheel loader. For this purpose there are at least onehydraulic working cylinder arranged in the wheel loader for lifting andlowering a lifting arm unit, on which a bucket or other type ofattachment or working tool is mounted for example forks. By use ofanother hydraulic working cylinder, the bucket can also be tilted orpivoted. Further hydraulic cylinders known as steering cylinders arearranged to turn the wheel loader by means of relative movement of afront and rear body part of the wheel loader.

To protect the combustion engine of a wheel loader from rapid changes inthe working conditions of the gearbox and the driving wheels it iscommon to provide the transmission line with a hydrodynamic torqueconverter or similar arranged between the combustion engine and thegearbox. The hydrodynamic torque converter provides an elasticity thatenables a very quick adaptation of the output torque to the changes inthe working conditions of the gearbox and the driving wheels of theloader. In addition, the torque converter provides an increased torqueduring particularly heavy working operations, e.g. during accelerationof the loader. However, these advantages are paid by high losses, sincethe elasticity and the increased torque provided by the torque converterare obtained by slipping between the impeller, turbine-wheel and thestator of the torque converter.

To utilize the advantages of a torque converter with respect toelasticity and torque increase for handling rapid changes in the workingconditions, at the same time as the advantages of a purely mechanicaltransmission is utilized with respect to efficiency (in principle 100%),it has been increasingly common in working machines of today tointroduce torque converters with a lock-up function. A lock-up functioncan provide a mechanical locking of the torque converter at a certainlow degree of slipping, i.e. the gear ratio of the torque converterbecomes fixed (1:1) at a certain low degree of slipping, e.g. at a lowdegree of slipping obtained during transportation speed. This maycertainly be an alternative for wheel loaders in some specificapplications.

However, the most typical application for wheel loaders is the so-calledshort-cycle load, in which the wheel loader moves materials between twoplaces near to each other, e.g. moves gravel from a heap to the loadingplatform of a nearby truck. The transportation distances in such cyclesare too short to let the torque converter reach the lock-up state.Moreover, a lock-up may not always be preferred since there is a stronginteraction between the hydraulic system and the transmission line,which implies that the combustion engine benefits from the elasticity ofthe torque converter to reduce the interaction of the transmission linewith the vehicle wheels. This is emphasized in modern wheel loaderswherein the combustion engine is utilized at lower rotational speeds dueto fuel economy reasons, giving the engine even greater difficulties torecover from sudden increases in working load.

One may summarize by saying that designers would actually prefer atorque converter with a lock-up function adapted for transportationpurposes. However, a lock-up function cannot be utilized in typicalshort-cycle loads or similar. Therefore a comparably rigid torqueconverter is chosen as the second best alternative. With a comparablyrigid torque converter it is possible to obtain a good fuel economy bothby utilizing a lower rotational speed for the combustion engine and byreducing any power consuming slipping in the torque converter. In thesame way as in a rigid spring a rigid torque converter reacts less on anouter load than a soft converter. Hence, a rigid torque converter givesa reduced degree of slipping compared to a soft torque converter and theother way around; a rigid converter provides certain torque increase ata lower degree of slipping compared to a soft converter.

However, in some phases of a typical short-cycle load a much softertorque converter is preferred or even needed instead of the comparablyrigid torque converter that is normally used. One such critical phase isproduced when the bucket of a wheel loader is emptied on a nearby truck.Here, the bucket is usually nearly completely raised as the wheel loaderapproaches the truck. At the same time the hydraulic lift and tiltfunctions are exercised to raise the bucket even further and to finallyemptying the bucket on the truck platform. In this situation it isdesirable to roll slowly forward towards the truck in a controlledmanner. However, a rigid torque converter will typically providetraction power for the driving wheels of such magnitude that the wheelloader rolls faster than desired even if the combustion engine isrunning on idle. This forces the operator of the loader to exercise thelifting, tilting and throttle controls to balance the lifting andtilting operations with the engine power (lifting and tilting mayrequire more throttle to create the necessary power), at the same timeas he has to exercise the brake to control the rolling speed. This is arather complicated operation which lowers the productivity even for moreexperienced operators. In addition, this has the potential to increasethe fuel consumption since the operator may choose to run the combustionengine at a higher rotation speed to meet the load from the hydraulicsystem while the forward rolling of the loader is controlled by thebrake pedal.

Another critical phase is produced when the bucket of a wheel loader isto be filled. Naturally, it is preferred that the bucket is filled in aquick and efficient manner. This is accomplished by the operator tryingto find the right balance between the bucket movement (controlled by thelifting and tilting functions) and the penetration the forward rollingcontrolled by the throttle pedal). Here, the traction forces from thewheels of the loader are in many situations counteracting the forcesfrom the moving bucket i.e. the tilt and lift movements). To accomplisha quick and efficient filling of the bucket and simultaneously handlingthe forces from the wheels and the bucket is a more or less complicatedtask depending on the characteristics of the wheel loader. Here, therigidness of the torque converter is an essential component.

Hence, both in the bucket emptying phase and in the bucket filling phaseit is preferred to utilize a soft torque converter. In the bucketemptying phase this enables an improved coordination of the bucket andwheel loader movements. In the bucket filling phase this enables animproved balancing of the forces created by the bucket and wheel loadermovements.

However, a soft torque converter is only preferred in such criticalphases of a short-cycle load as those described above and similar. Inthe other phases of a short-cycle load a rigid torque converter ispreferred for the reasons of performance and fuel economy.

Considering the above there is clearly a need for a working machine witha transmission line comprising a transmission unit {e.g. a torqueconverter) where the working machine is provided with an ability toovercome the shortcomings of known transmission units being lesssuitable for at least some working conditions of the working machine.

It is desirable to provide a method of the kind referred to in theintroduction, which creates conditions for a more effective operation ofthe working machine.

According to an aspect of the present invention, a method is providedfor operating a working machine provided with: a power source and aplurality of driving wheels; a working hydraulic system comprising atleast one hydraulic pump powered by the power source for moving animplement on the working machine and/or for steering the workingmachine; a transmission line arranged between the power source and thedriving wheels for transmitting torque from the power source to thedriving wheels; and a transmission unit arranged in the transmissionline for reducing the mechanical interaction between the power sourceand the driving wheels.

The method is characterized by the steps of:

-   -   detecting at least one operational parameter indicative of a        working condition of the working machine,    -   determining if the characteristic of the transmission unit        should be altered on the basis of a magnitude of the detected        operational parameter,    -   altering the characteristic of the transmission unit if it is        determined to be required.

Altering the characteristic of the transmission unit by means of theabove method provides a working machine with an improved ability toovercome the shortcomings of known transmission units being lesssuitable for at least some working conditions of the working machine.

This is particularly so if the working condition determines apredetermined working operation with the implement, since thisconstitutes a typical situation in which the need for altering thecharacteristic of the transmission unit can arise. Here, it may e.g. beadvantageous to altering the characteristic of the transmission unit soas to reduce the mechanical interaction between the power source and thedriving wheels even more leaving the driving wheels with less power andthe hydraulic system with an increased power.

It is preferred that the characteristic is altered by means of at leastone electric machine, since this enables a flexible and compact design.An electric machine can also be powered by means of a plurality of powersources (e.g. batteries, generators, fuel cells etc), which provides anincreased freedom in the design. Moreover, electric machines react faston commands providing an improved control over the alteration of thecharacteristic of the transmission unit.

It is particularly preferred that at least first electric machine isarranged downstream the transmission unit for subtracting torque fromthe downstream side of the transmission unit and converting this torqueto electric energy. This provides energy that can be used for alteringthe characteristic of the transmission unit. Hence, it is not necessaryto have an auxiliary power source and the requirements on a possibleauxiliary power source can be relaxed. Typically, the energy shouldotherwise have been supplied to the driving wheels. However, the workingconditions at which the characteristic of the transmission unit isadvantageously altered are typically admitting that energy can bewithdrawn from the driving wheels.

In addition it is preferred that at least a second electric machine isarranged upstream the transmission unit for receiving electric energyfrom the first electric machine and converting at least a part of thisenergy to torque that is added to the upstream side of the transmissionunit. The use of a first and a second electric machine in this mannerprovides an excellent control over the alteration of the characteristicof the transmission unit.

It is also desirable to provide a working machine of the kind referredto in the introduction, which working machine enables a more effectiveoperation of the working machine.

According to another aspect of the present invention, a working machineis provided with: a power source and a plurality of driving wheels; aworking hydraulic system comprising at least one hydraulic pump poweredby the power source for moving an implement on the working machineand/or for steering the working machine; a transmission line arrangedbetween the power source (120) and the driving wheels for transmittingtorque from the power source to the driving wheels; and a transmissionunit arranged in the transmission line for reducing the mechanicalinteraction between the power source and the driving wheels.

In addition the working machine comprises:

-   -   at least one detecting unit for detecting at least one        operational parameter indicative of a working condition of the        working machine,    -   at least one control unit for determining if the characteristic        of the transmission unit should be altered on the basis of a        magnitude of the detected operational parameter,    -   at least one torque-modifying unit controlled by said control        unit for altering the characteristic of the transmission unit if        it is determined to be required.

The working machine displays the same or similar advantages as themethod described above.

Further advantages and advantageous features of the invention aredisclosed in the following description.

DEFINITIONS

The term “electric machine” should be understood as a term for anelectric motor and/or generator. The electric machine can be driven byelectricity to supply an output torque to a shaft or be mechanicallydriven by receiving torque on a shaft for producing electricity.

The term “transmission unit” comprises hydraulic clutches, bothhydrodynamic clutches such as torque converters and hydrostaticclutches, as well as mechanical clutches. Thus, “transmission unit”comprises both torque converters which can increase the torque andordinary skid clutches without ability to increase the torque.

The term “case load” refers to the working condition for a specifictransmission unit at a given point in time. The “case load” at a givenpoint in time can e.g. be described by means of the input torque Tjn andthe input rotational speed njn applied to the transmission unit inconjunction with the output torque Tout and the output rotational speednout received from the transmission unit at that point in time.

The term “driving wheels” is meant to comprise vehicle wheels for directengagement with the ground as well as vehicle wheels for driving aground engaging member, such as tracks, crawlers or similar.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed description of the present invention is given below withreference to a plurality of exemplifying embodiments as illustrated inthe appended figures, in which:

FIG. 1 is a lateral view illustrating a wheel loader having a bucket forloading operations, and a working hydraulic system for operating thebucket and steering the wheel loader,

FIG. 2 is a schematic illustration of a working hydraulic system for awheel loader,

FIG. 3 is a schematic illustration of i.a. transmission line of a wheelloader according to an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 is an illustration of an exemplifying wheel loader 1 having animplement 2 in the form of a bucket 3. The bucket 3 is arranged on anarm unit 4 for lifting and lowering the bucket 3. The bucket 3 can alsobe tilted or pivoted relative to the arm unit 4. For this purpose thewheel loader 1 is provided with a working hydraulic system comprising atleast one hydraulic pump (not shown in FIG. 1) and working cylinders 5a, 5 b, 6 for lifting and lowering of the arm unit 4, and for tilting orpivoting the bucket 3. In addition, the working hydraulic systemcomprises working cylinders 7 a, 7 b for turning the wheel loader 1 bymeans of relative movement of a front body 8 and a rear body 9. Thesefeatures of the wheel loader 1 and variations thereof are well known tothose skilled in the art and they need no further explanation.

FIG. 2 is a schematic illustration of an exemplifying working hydraulicsystem 140. The embodiment illustrated in FIG. 2 comprises two workingcylinders known as lifting cylinders 5 a, 5 b. The lifting cylinders 5a, 5 b are arranged for lifting and lowering the arm unit 4. A furtherworking cylinder known as tilting cylinder 6 is arranged for tilting-inor tilting-out the bucket 3 relative to the arm unit 4. In addition, twoworking cylinders known as the steering cylinders 7 a, 7 b are arrangedfor steering the wheel loader 1. Three hydraulic pumps 142, 144, 146supply the hydraulic cylinders with hydraulic oil. An operator of thewheel loader 1 can control the working cylinders by means of instrumentsconnected to a control unit (not shown). Preferably the cylinders 5 a, 5b, 6, 7 a and 7 b schematically illustrated in FIG. 2 correspond thecylinders 5 a, 5 b, 6, 7 a and 7 b shown in FIG. 1.

FIG. 3 is a schematic illustration of i.a. a transmission line 110 of awheel loader 1 according to an embodiment of the present invention. Theinternal combustion engine 120 of the wheel loader 1 is arranged at oneend of the transmission line 110, whereas the driving wheels 130 of thewheel loader 1 are arranged at the other end of the transmission line110. It follows that the internal combustion engine 120 is arranged tosupply torque to the driving wheels 130 of the wheel loader 1 via thetransmission line 110. It is preferred that the transmission line 110comprises a gearbox 118 for varying the speed of the driving wheels 130of the wheel loader 1 and for changing between forward and backwarddriving direction of the wheels 130. The gearbox 118 may e.g. be anautomatic gearbox implying that there must not necessarily be a clutch(not shown) between the gearbox 118 and the driving wheels 130, which iscommon in the case of a manual gearbox.

As discussed above in the background to the present invention thetransmission line 110 of a wheel loader is usually provided with atransmission unit 114 for reducing the mechanical interaction betweenthe internal combustion engine 120 and the driving wheels 130, i.e. forproviding slipping or skidding or even for temporally disengaging theinternal combustion engine 120 from the driving wheels 130. The mainpurpose is to protect the combustion engine 120 from rapid changes inthe working conditions of the gearbox 118 and the driving wheels 130.

The transmission unit 114 is preferably a hydraulic clutch of the typecalled hydrodynamic torque converter. As is well known, a torqueconverter is adapted to increase the input torque applied to theconverter and the output torque can be in the interval of e.g. 1-3 timesthe input torque. The torque converter may also have a free wheelfunction and/or a lock-up function providing a direct operation withoutany increased torque. In case of a lock-up function it is preferred thatthe lock-up state provides a fixed transmission ratio of substantially1:1. It should be added that alternative embodiments of the presentinvention may comprise a transmission unit 114 in the form of a skidclutch or similar without any torque-increasing ability. The skid clutchcould be a hydraulic clutch as well as a mechanic clutch.

The exact position of the transmission unit 114 within the transmissionline 110 is not decisive. However, it is preferred that the transmissionunit 114 is positioned after or down streams the combustion engine 120and before or up streams the gearbox 118.

In addition, the transmission line 110 of the wheel loader 1 is providedwith a power transferring unit 116 for driving the hydraulic pumps 142,144, 146 of the working hydraulic system 140 so as to enable the liftingand steering operations as mentioned before. The power transferring unit116 may e.g. be gear wheels or some other suitable means arranged tointeract with the transmission line 110 for transferring power from thecombustion engine 120 to the hydraulic pumps 142, 144, 146. The powertransferring unit 116 is preferably arranged to interact with thetransmission line 110 in a position upstream the gear box 118 and morepreferably in a position between the internal combustion engine 120 andthe transmission unit 114.

It should be added that the combustion engine 120 can be replaced byother power sources, e.g. a power sources in the form of a gas turbineor even a fuel cell arrangement. In addition, the power transferringunit 116 may be fully or at least partly replaced by other powertransferring means based on hydraulic or electric principles. Forexample, the hydraulic pumps 142, 144, 146 may be powered by means ofelectric motors receiving power from the combustion engine 120 via agenerator arrangement or similar.

As can be seen in FIG. 3 the transmission line 110 is provided with atleast two electric machines 112 a, 112 b or similar torque-modifyingunit or units for adding and/or subtracting torque to and/or from thetransmission line 110. The electric machines 112 a, 112 b are arrangedto operatively adapt the characteristic of the transmission unit 114 andparticularly to adapt the rigidness of the transmission unit 114depending on the working condition of the wheel loader 1. Preferably, afirst electric machine 112 a is arranged in a suitable positiondownstream the transmission unit 114 (i.e. at the gear box side of thetransmission unit 114), whereas a second electric machine 112 b isarranged in a suitable position upstream the transmission unit 114 (Ae.at the combustion engine side of the transmission unit 114). Moreprecisely, the first electric machine 112 a is preferably arranged in aposition between the transmission unit 114 and the gearbox 118, and thesecond electric machine 112 b is preferably arranged in a positionbetween the internal combustion engine 120 and the transmission unit114. Naturally, other alternative positions are conceivable. Theelectric machines 112 a, 112 b and the torque converter 114 are coupledso that torque can be exchanged between the first electric machine 112 aand the input shaft of the transmission unit 114, and between the secondelectric machine 112 b and the output shaft of the transmission unit114. The first electric machine 112 a should preferably be able tooperate in at least two quadrants, i.e. as generator in both clockwiseand counter-clockwise direction of rotation. The second electric machine112 b should preferably be able to operate in at least one quadrant,i.e. as motor in at least one direction of rotation.

The electric machines 112 a, 112 b in FIG. 3 are electrically connectedto each other via a transmission-control unit 200 or a similar controlunit being arranged to control the machines 112 a, 112 b for adaptingthe characteristic of the transmission unit 114. This is preferablyaccomplished by controlling the motor and generator abilities of theelectric machines 112 a, 112 b. The transmission-control unit 200 ispreferably implemented as a hardware unit provided with the appropriatecircuitry and software needed to accomplish the required functions, e.g.circuitry for processing and storing; and software for executing andcontrolling any required processing and storing. It should be emphasisedthat some embodiments of the present invention may have a very simpletransmission-control unit 200 comprising a simple on/off switchingfunction for connecting and disconnecting the electric machines 112 a,112 b, e.g. on a command from the operator of the wheel loaderexercising a push button or similar. However, other embodiments may havea more sophisticated transmission-control unit 200 provided withsubstantial processing capabilities and advanced switching functions forcontrolling the motor and generator abilities of the electric machines112 a, 112 b depending on algorithms working on data received fromsensors 113 a, 1136 arranged within the wheel loader 1 and preferablyconnected to the transmission-control unit 200. For this purpose thetransmission-control unit 200 may e.g. use data in the form of sensed,measured or even calculated input torque Tin and input rotational speednin applied to the torque converter 114, and output torque Tout andoutput rotational speed nout received from the torque converter 114. Itshould be emphasised that it is well known by those skilled in the artthat input torque Tin and output torque Tout can be calculated byknowing the characteristic of the transmission-control unit 200 and theinput rotational speed njn and the output rotational speed nout. Sensorsfor measuring torque and rotational speed are well known to thoseskilled in the art. Likewise, a wide range of commercially availablecontrol units with substantial processing capabilities and advancedswitching functions for controlling electric machines are well known bythose skilled in the art and they need no further description.

The transmission-control unit 200 is arranged to operatively connect theelectric machines 112 a, 112 b to each other so that the first electricmachine 112 a operates as a generator and so that the second electricmachine 112 b operates as a motor. In particular, thetransmission-control unit 200 is arranged to operatively connect theelectric machines 112 a, 112 b so that the electric energy generated bythe first electric machine 112 a is used in the second electric machine112 b. This enables the first electric machine 112 a downstream thetransmission unit 114 to subtract torque from the downstream side of thetransmission unit 114 and to convert this torque to electric energy,whereas it enables the second electric machine 112 b upstream thetransmission unit 114 to receive the electric energy produced by thefirst electric machine 112 a and to convert this energy to torque thatis added to the upstream side of the transmission unit 114. In this waythe internal combustion engine 120 will experience a transmission unitwith a softer characteristic compared to the actual and unaffectedcharacteristic of the used transmission unit 114.

There are several strategies for adapting the characteristic of thetransmission unit 114 as can be illustrated by the exemplifyingembodiments describe below.

In an embodiment of the present invention it is preferred thatsubstantially all electric energy produced by the first electric machine112 a is transferred by the transmission-control unit 200 to the secondelectric machine 112 b. In other words, substantially all the torquesubtracted by the first electric machine 112 a from the downstream sideof the transmission unit 114 is added by the second electric machine 112a to the upstream side of the transmission unit 114.

In another embodiment of the present invention it is preferred that adetermined portion of the electric energy produced by the first electricmachine 112 a is transferred by the transmission-control unit 200 to thesecond electric machine 112 b. In other words, a determined portion ofthe torque being subtracted the by first electric machine 112 a from thedownstream side of the transmission unit 114 is added to the upstreamside of the transmission unit 114.

In still another embodiment of the present invention it is preferredthat a variable amount of the electric energy produced by the firstelectric machine 112 a is transferred by the transmission-control unit200 to the second electric machine 112 b. In other words, a variableamount of the torque being subtracted from the downstream side of thetransmission unit 114 is added to the upstream side of the transmissionunit 114.

The feedback of a variable amount of torque makes it possible to e.g.adopt a strategy wherein the torque subtracted from the downstream sideof the transmission unit 114 is added to the upstream side of thetransmission unit 114 in an amount that maintains the input torque fromthe combustion engine 120 to the transmission unit 114 at asubstantially constant level. Naturally, this may only be accomplishedto the extent and within the limits the subtracted torque is sufficientto maintain a substantially constant input torque. However, the supportfrom an additional power source may extend the limits within which theinput torque from the combustion engine 120 can be maintainedsubstantially constant. This may e.g. be accomplished by means of anelectric storage means 210 providing additional electric energy to thesecond electric machine 112 b. An electric storage means 210 isillustrated in FIG. 3 and it will be more thoroughly discussed below.

In addition, the feedback of a variable amount of torque makes itpossible to e.g. adopt a strategy wherein the output torque from thetransmission unit 114 is maintained at a substantially constant level bysubtracting a variable amount of torque from the downstream side of thetransmission unit 114 and add this torque to the upstream side of thetransmission unit 114. Naturally, this may be most feasible when thetorque on the output side of the transmission unit 114 is provided withan increasing torque, e.g. due to an increased input torque to thetransmission unit 114 from the combustion engine 120. The other wayaround, this may not be feasible when the torque on the output side ofthe transmission unit 114 is provided with a declining torque, e.g. dueto a declining input torque to the transmission unit 114 from thecombustion engine 120. However, an external power source, e.g. abattery, may certainly change this.

Moreover, the feedback of a variable amount of torque makes it possibleto use a first transmission unit 114 having a first rigid characteristicfor emulating a second transmission unit having a second softercharacteristic.

To accomplish this it is necessary to know the characteristics of thesoft transmission unit to be emulated. This characteristic can e.g. berepresented by means of a suitable lookup table that is built onempirical measurements in laboratory conditions and/or by sampling dataduring real life use.

An exemplifying table representing the characteristics of a transmissionunit to be emulated may e.g. comprise the following variables:

Tin=input torque to the transmission unit

nin=input rotational speed to the transmission unit

Tout=output torque from the transmission unit

nout=output rotational speed from the transmission unit

This illustrates that a certain torque Tjn and a certain rotationalspeed njn being inputted to the transmission unit correspond to acertain torque Tout and a certain rotational speed njn being outputtedfrom the soft transmission unit. Such a table can comprise all relevantcases of load for a certain transmission unit, e.g. measured inlaboratory conditions and/or sampled in real life use.

Alternatively or additionally, the characteristic of a transmission unitmay be described by means of one or several mathematical expressions orsimilar. For example, the simplified converter model given by the twoexemplifying mathematical relations 1 and 2 below is commonly used todescribe the characteristic of a transmission unit in the form of ahydrodynamic torque converter. Naturally, depending on the nature of thetransmission unit there are clearly other mathematical expressions orsimilar that can be used to describe the characteristic of a particulartransmission unit.

The simplified converter model mentioned above is based on two simpleempirical relations.

$\begin{matrix}{{T_{in} = {{k(v)}n_{in}^{2}}},{{{where}\mspace{14mu}{k(v)}} = \frac{T_{{in} \cdot {ref}}v}{n_{{in} \cdot {ref}}^{2}}}} & (1) \\{T_{out} = {{\mu(v)}T_{in}}} & (2)\end{matrix}$

wherein

Tin represents the present input torque

Tin.ref represents the input torque at a determined reference inputrotational speed

Tout represents the present output torque

nin represents the present input rotational speed

nin.ref represents a determined reference input rotational speed

k(v) represents the absorption factor for the converter in question atdifferent input and output rotational speeds

μ(v) represents the amplifying factor for the converter in question atdifferent input and output rotational speeds

v represents the input rotational speed njn divided by the outputrotational speed nout.

Values for the factors k(v) and μ(v) for a certain torque converter canbe obtained by running the converter at a reference input rotationalspeed njn,ref (e.g. at 1000 rpm) while the output rotational speed isvaried. The simplified converter model described by the relations 1, 2above and the manner of obtaining the factors k(v) and μ(v) are wellknown facts to those skilled in the art and they need no furtherexplanation.

Considering the present invention and the above discussion of thecharacteristic of a transmission unit it should be clear that a firstrigid transmission unit can be used to emulate a second softtransmission unit by subtracting a first amount of torque from thedownstream side of the rigid transmission unit and by adding a secondamount of torque to the upstream side of the rigid transmission unit.The amount of torque subtracted from the downstream side of the rigidtransmission unit and the amount of torque added to the upstream side ofthe rigid transmission unit should then be determined so that thecurrent case load is adapted to a case load that is determined by thecharacteristic of a softer transmission unit.

As an example we can use the two relations Tin=k(v)nin² and Tout=μ(v)Tindescribed above and the known factors krigid(v) and μrigid(v) for arigid torque converter to calculate the input and output torques Tjnrigid and Tout rigid for the rigid torque converter at a specific inputand output rotational speed nin, nout. Likewise, given the known factorsksoft(v) and μsoft(v) for a softer torque converter we can alsocalculate the corresponding input and output torques Tin_soft andTout_soft for the soft converter at the same input and output rotationalspeeds njn, nout. Hence, by maintaining the same input and outputrotational speed njn, nout and by subtracting a first amount of torquefrom the downstream side and adding a second amount of torque to theupstream side of the rigid torque converter so that the new input andoutput torques equals the input and output torques Tin_soft andTout_soft it is possible to adapt the current case load for the rigidtorque converter to a corresponding case load for the soft torqueconverter. A combustion engine connected to the input shaft of the rigidtorque converter will then experience the characteristic of the softconverter instead of the characteristic of the unaffected rigidconverter.

The same can be accomplished by using a look-up table, e.g. theexemplifying look-up table described above or similar defining thecharacteristic of a softer torque converter. Knowing the current inputand output rotational speeds njn, nout for a rigid torque converter itis be possible to find the same or at least similar pair of input andoutput rotational speeds in the look-up table for the soft convertertogether with the input and output torques Tin_soft and Tout soft forthat converter at that input and out put speed. By maintaining the sameinput and output rotational speed nin, nout and by subtracting a firstamount of torque from the downstream side and adding a second amount oftorque to the upstream side of the rigid torque converter so that thenew input and output torques equals the input and output torquesTin_soft and Tout_soft it is possible to adapt the current case load forthe rigid torque converter to a corresponding case load for the softtorque converter.

Naturally, it is preferred that the characteristic of the transmissionunit 114 is adapted only for those phases in the working condition of awheel loader that requires a softer transmission unit. As previouslydescribed, the bucket filling phase and the bucket emptying phase in ashort-cycle load are examples of such phases.

Both the bucket filling phase and the bucket emptying phase in ashort-cycle load are performed while the wheel loader is running on thelowest gear or at least on a low gear. Hence, in an embodiment of thepresent invention it is preferred to adapt the characteristic of thetransmission unit 114 when the wheel loader is running on the lowestgear or at least on a low gear.

Similarly, at least the bucket emptying phase in a short-cycle load istypically performed while the operator exercises the brakes toaccomplish a slow forward movement for the wheel loader. Hence, in anembodiment of the present invention it is preferred to adapt thecharacteristic of the transmission unit 114 when the operator exercisesthe brakes.

In addition, a push button or some other control can be used formanually activating and/or selecting the desired strategy for adaptingthe characteristic of the transmission unit 114.

It should be added that an embodiment of the present invention comprisesan additional power source in the form of an electric energy storagemeans 210 for receiving electric energy from the first electric machine112 a and providing electric energy to the second electric machine 112b. The electric storage means 210 makes it possible to at leasttemporary provide the second electric machine 112 b with an amount ofelectric power that exceeds the amount currently produced by the firstelectric machine 112 a when subtracting torque from the downstream sideof the transmission unit 114. In addition, the electric storage means210 may be provided with charging electric energy from the firstelectric machine 112 a, e.g. when the amount of electric power currentlyproduced by the first electric machine 112 a exceeds the amountcurrently required for the second electric machine 112 b. This providesan improved flexibility in reducing the rigidness of the transmissionunit (114) by means of the first and second electric machines 112 a, 112b as described above. The electric storage means 210 may e.g. be abattery or a super capacitor or some other suitable electric storagemeans.

Although the exemplifying working hydraulic system 140 illustrated inFIG. 2-3 has three hydraulic pumps 142, 144, 146 other embodiments mayhave one, two, four or more hydraulic pumps. In a preferred embodimentof the invention the working machine has at least two implement and/orsteering functions, and at least one said hydraulic pump is arranged foreach implement and/or steering function.

As described in connection to the FIG. 1, the working machine 1 can havean implement 2 in the form of a bucket 3 which is operated by means ofthe working hydraulic system 140. However, it should be emphasised thatother implements are usable. When applying the invention on a workingmachine such as an articulated hauler or a truck, the implement caninstead be for example a dump body. Usually a hydraulic pump and workingcylinders are used for the operation of the dump body during the dumpingmovement.

It is to be understood that the present invention is not limited to theembodiments described above and illustrated in the drawings; rather, theskilled person will recognize that many changes and modifications may bemade within the scope of the appended claims.

The invention claimed is:
 1. A method for operating a working machineprovided with: a power source and a plurality of driving wheels; aworking hydraulic system comprising at least one hydraulic pump poweredby the power source for moving an implement on the working machineand/or for steering the working machine; a transmission line arrangedbetween the power source and the driving wheels for transmitting torquefrom the power source to the driving wheels; and a transmission unitarranged in the transmission line for reducing mechanical interactionbetween the power source and the driving wheels, comprising the stepsof: detecting at least one operational parameter indicative of a workingcondition of the working machine, determining if a characteristic of thetransmission unit should be altered on the basis of a magnitude of thedetected operational parameter, altering the characteristic of thetransmission unit if it is determined to be required, wherein theworking condition determines a predetermined working operation with theimplement.
 2. A method according to claim 1, comprising the steps of:temporarily altering the characteristic of the transmission unit duringthe predetermined working operation.
 3. A method according to claim 1,comprising the steps of: altering the characteristic of the transmissionunit by altering its tendency to slip.
 4. A method according to claim 1,comprising the steps of: adding and/or subtracting torque upstreamand/or downstream of the transmission unit by using at least onetorque-modifying unit in case the characteristic of the transmissionunit should be altered.
 5. A method according to claim 4, comprising thesteps of: using at least one electric machine for adding and/orsubtracting the torque.
 6. A method according to claim 5, comprising thesteps of: using at least a first electric machine arranged downstreamthe transmission unit for subtracting torque from the downstream side ofthe transmission unit and converting this torque to electric energy. 7.A method according to claim 6, comprising the steps of: using at least asecond electric machine arranged upstream the transmission unit forreceiving electric energy from the first electric machine and convertingat least a part of this energy to torque that is added to the upstreamside of the transmission unit.
 8. The method according to claim 7,comprising using the electric machines so as to transfer substantiallyall electric energy produced by the first electric machine to the secondelectric machine.
 9. The method according to claim 7, comprising usingthe electric machines so as to transfer a determined portion of theelectric energy produced by the first electric machine to the secondelectric machine.
 10. The method according to claim 7, wherein theelectric machines transfer a variable amount of the electric energyproduced by the first electric machine to the second electric machine.11. The method according to claim 10, wherein the amount of electricenergy produced by the first electric machine and the amount of electricenergy transferred to the second electric machine are determined so thatthe input torque from the power source to the transmission unit ismaintained at a substantially constant level.
 12. The method accordingto claim 10, wherein the amount of electric energy produced by the firstelectric machine and the amount of electric energy transferred to thesecond electric machine are determined so that the output torque fromthe transmission unit is maintained at a substantially constant level.13. The method according to claim 10, wherein the amount of electricenergy produced by the first electric machine and the amount of electricenergy transferred to the second electric machine are determined so thatthe current case load determined by the characteristic of the currenttransmission unit is adapted to another case load determined by thecharacteristic of a softer transmission unit.
 14. A method according toclaim 5, comprising the steps of: using a control unit for controllingat least one electric machine.
 15. The method according to claim 1,wherein a second machine is at least partly provided with electricenergy from an electric energy storage means.
 16. The method accordingto claim 15, wherein a first electric machine provides charging electricenergy to the storage means.
 17. A method according to claim 1,comprising the steps of: adding torque to the transmission line.
 18. Amethod according to claim 1, comprising that: the power source ismechanically connected to the working hydraulic system.
 19. A methodaccording to claim 1, comprising that: the power source is an internalcombustion engine.
 20. The method according to claim 1, comprising thatthe operated working machine is a wheel loader.
 21. A working machineprovided with: a power source and a plurality of driving wheels; aworking hydraulic system comprising at least one hydraulic pump poweredby the power source for moving an implement on the working machineand/or for steering the working machine; a transmission line arrangedbetween the power source and the driving wheels for transmitting torquefrom the power source to the driving wheels; and a transmission unitarranged in the transmission line for reducing the mechanicalinteraction between the power source and the driving wheels, comprising:at least one detecting unit for detecting at least one operationalparameter indicative of a working condition of the working machine, atleast one control unit for determining if the characteristic of thetransmission unit should be altered on the basis of a magnitude of thedetected operational parameter, at least one torque-modifying unitcontrolled by the control unit for altering the characteristic of thetransmission unit if it is determined to be required, wherein thecontrol unit is arranged to operatively determine the working conditionwith reference to a predetermined working operation with the implement.22. A working machine according to claim 21, wherein: the control unitis arranged to temporarily alter the characteristic of the transmissionunit during the predetermined working condition.
 23. A working machineaccording to claim 21, wherein: the control unit is arranged tooperatively alter the characteristic of the transmission unit byaltering its tendency to slip.
 24. A working machine according to claim21, wherein: the at least one torque-modifying unit is arranged tooperatively add and/or subtract torque upstream and/or downstream of thetransmission unit in case the characteristic of the transmission unitshould be altered.
 25. A working machine according to claim 24, wherein:the torque-modifying unit comprises at least one electric machine beingarranged to operatively add and/or subtract the torque.
 26. A workingmachine according to claim 25, wherein: at least a first electricmachine is arranged downstream the transmission unit for operativelysubtract torque from the downstream side of the transmission unit andconvert this torque to electric energy.
 27. A working machine accordingto claim 26, wherein: at least a second electric machine is arrangedupstream the transmission unit for operatively receive electric energyfrom the first electric machine and convert this energy to torque thatis added to the upstream side of the transmission unit.
 28. A workingmachine according to claim 25, wherein: at least one electric machine isarranged to be operatively controlled by the control unit.
 29. A workingmachine according to claim 21, wherein: the transmission line isarranged to receive and/or provide torque for operatively altering thecharacteristic of the transmission unit if it is determined to berequired.
 30. A working machine according to claim 21, wherein a firstelectric machine is arranged between the transmission unit and agearbox, or between the transmission unit and the driving wheels.
 31. Aworking machine according to claim 30, wherein a second electric machineis arranged between an engine and the transmission unit.
 32. A workingmachine according to claim 21, wherein the transmission unit is ahydrodynamic torque converter; or a hydraulic or mechanic skid clutch.33. A working machine according to claim 21, wherein: the power sourceis mechanically connected to the working hydraulic system.
 34. A workingmachine according to claim 21, wherein: the power source is an internalcombustion engine.
 35. A working machine according to claim 21, whereinthe working machine is a wheel loader.
 36. A working machine accordingto claim 21, wherein the working machine has at least two implementand/or steering functions, and in that at least one hydraulic pump isarranged for each implement and/or steering function.
 37. A workingmachine according to claim 21, wherein the working machine comprisesthree hydraulic pumps; a first hydraulic pump being arranged to providea lifting and lowering function of the implement, a second hydraulicpump being arranged to provide a tilting function of the implement, anda third hydraulic pump being arranged to provide the steering functionof the working machine.